Semiconductor wafer (“wafer”) fabrication often includes exposing a wafer to a plasma to allow the reactive constituents of the plasma to modify the surface of the wafer, e.g., remove or etch material from unprotected areas of the wafer surface. The wafer characteristics resulting from the plasma fabrication process are dependent on the process conditions, including the plasma density profile across the wafer surface. Additionally, because an amount of reaction between the plasma and a particular portion of the wafer surface is directly proportional to the plasma density over the particular portion of the wafer surface, variations in the plasma density profile can result in center-to-edge wafer uniformity problems. Such center-to-edge wafer uniformity problems can adversely affect a die yield per wafer.
Objectives in wafer fabrication include optimizing a die yield per wafer and fabricating each wafer of a common type in as identical a manner as possible. To meet these objectives, it is desirable to control the plasma density profile across the wafer and thereby control the etch uniformity across the wafer. Previous plasma processing techniques have attempted to establish a process window for a particular plasma chamber configuration that will yield an acceptable etch uniformity across the wafer. Such a process window is traditionally defined by parameters such as a pressure range, a gas flow range, a wafer temperature range, and a power level range. Experience indicates that a given process window is limited by the physical hardware configuration of the plasma chamber. Therefore, a plasma etching operation that falls outside of the process window achievable with a given plasma chamber may require the use of an alternate plasma chamber having a different physical hardware configuration and correspondingly different process window.
Procurement and maintenance of multiple plasma chambers having different physical hardware configurations in an effort to increase an overall plasma process window capability can be prohibitively expensive. Therefore, a solution is sought to expand the process window capability of a given plasma chamber.